This invention relates to a carrier synchronisation device for determining and compensating for differences in synchronisation between a local carder and a received modulated signal to be demodulated by the local carrier generated by said device, the device comprising:
means for determining an error signal .epsilon.(t) representative of a synchronisation error between the received modulated signal and the local carrier,
a local oscillator which generates the local carrier under control of a control signal u(t),
at least two low-pass filters connected in parallel between said means and the local oscillator, the two filters filtering the error signal and providing different loop bandwidths and
a lock mode detector, which supplies a selection signal for selecting one of the filters, the output of the selected filter supplying said control signal u(t).
The invention also relates to the use of this device for the synchronisation of signals transmitted in accordance with an OFDM (Orthogonal Frequency Division Multiplex) standard.
Such a synchronisation device is described, for example, in the JP KOKAI-60 1673 1. That document describes a circuit formed by a phase-locked loop which determines an error signal representative of a phase error, which error signal can be filtered either by a first filter or by a second filter, which filters have different pass bands, in a manner such that the error signal thus filtered produces a control signal which controls a local oscillator. The signal generated by this local oscillator is combined with the received signal in order to generate the error signal. A discriminator circuit detecting a lock-in mode or a capturing mode determines which of the two filters is to be put into operation.
How the discriminator circuit operates is not revealed. It is apparent only that the device performs a phase detection and that the discriminator circuit selects the mode of operation by a phase-related measurement.
However, there are also situations in which allowance is to be made for frequency differences. On the other hand, the discriminator circuit is fixed and cannot adapt itself automatically to operation in the capturing mode or in the lock-in mode.
A special use of the device is in the reception of signals transmitted in accordance with an orthogonal frequency division multiplex (OFDM) method, transmitted particularly by electromagnetic waves, by cable or by satellite, for example, for use in digital television. For uses such as the transmission of television pictures by OFDM modulation, phase detection is ineffective. Moreover, for large-scale uses the device should be not only robust and accurate but also inexpensive.
The technique of transmitting N orthogonal frequencies by frequency division multiplexing consists of splitting up the information to be transmitted by dividing it over a large number of elementary channels having a low bit rate 1/T, T being the bit period. Thus, a highly selective wide-band channel is converted into a large number of non-selective elementary channels. Since these elementary channels together form a wide-band channel, fading during transmission is unlikely to affect the entire channel simultaneously. This technique also enables intersymbol interference to be reduced.
Each elementary channel has a corresponding frequency, the frequencies as a whole being symmetrical about a carrier frequency. Since the use of selective filters during reception is difficult, it is preferred to allow the spectra to overlap but requirements are imposed on the orthogonality between the frequencies in order to eliminate intersymbol interference at the sampling instants. The complete spectrum of an OFDM signal will resemble a rectangular spectrum.
During reception the received signals should be demodulated and subsequently be decoded in order to recover the original information. For this demodulation a local oscillator is used whose frequency should be locked to the received carrier frequency.